Hydrocarbons with medium chain lengths are on the one hand essential raw materials for the chemical industry, on the other hand there is also a need for particularly oxygen-free hydrocarbons with medium chain lengths as an important building block for kerosene for use in aviation, for example as “drop-in aviation fuel” and here in particular in the form of biokerosene. Biokerosene is produced according to the prior art for example by hydrogenation of bio-based oils. According to the prior art, it is also known that by reacting fats, oils or fatty acids over porous catalysts, it is possible to obtain either liquid hydrocarbons or else to generate gaseous hydrocarbons in a targeted manner.
For example, DE 103 27 059 A1 and EP 1 724 325 A1 disclose methods in which fat-containing starting materials are brought into contact with an activated carbon fixed bed at elevated temperatures in a reactor. Here, a decarboxylation of the fatty acids present takes place, giving rise to liquid hydrocarbons. In order to arrive at gaseous hydrocarbons, the process parameters and optionally also the processing of the starting material can be modified such that a degradation of the starting materials to gaseous, short-chain hydrocarbons takes place. Increased formation of hydrocarbons with medium chain lengths, however, is not described in the cited documents.
Using the Fischer-Tropsch process, it is possible to produce hydrocarbons with any desired chain length from fat-containing starting materials; however, in this process, the starting material is firstly degraded to C1 segments, from which subsequently hydrocarbons with different chain lengths have to be built up again. This process is accordingly associated with considerable energy expenditure and, moreover, is only economically feasible on a very large scale.
It is therefore an object of the present invention to overcome the disadvantages of the prior art and to provide a method for converting oxygen-containing hydrocarbon compounds or mixtures of oxygen-containing hydrocarbon compounds into hydrocarbons with medium chain lengths. It is a further object of the present invention to provide a method for producing hydrocarbon mixtures that is suitable for producing biokerosene.
At least one of these objects is achieved by the subject matter of the independent claims. Further embodiments and developments are the subject matter of independent claims and are also evident from the description below.
A method for producing pure hydrocarbons with medium chain lengths or hydrocarbon mixtures with an increased fraction of hydrocarbons with medium chain lengths comprises the following steps:
A) Firstly a starting material is provided which comprises at least 50% by weight of unsaturated oxygen-containing hydrocarbon compounds. These unsaturated hydrocarbon compounds have at least in part structural units which satisfy the following specification (these structural units are referred to hereinbelow as olefin fragments since they are responsible, at least in part, for the unsaturated character of the hydrocarbon compounds). The olefin fragments have the formula —C1-CxH2x—CH═CH—CyH2y+1 (formula I). The carbon atom C1 in this olefin fragment marks the transition to a heteroatom. Accordingly, the carbon atom C1 is saturated with at least one substituted or unsubstituted heteroatom and optionally also hydrogen. Often, an oxygen atom will be bonded to the carbon atom C1 and this again is then bonded to a carbon atom (such as for example in a fatty acid ester, for example a glyceride) or, together with the carbon atom C1, is a part of a carboxylate group (such as e.g. in a fatty acid). The indices x and y in the given formulae are integers, where x is greater than 1 and y is greater than 0. In other words, it is neither an olefin fragment with a terminal double bond, nor an olefin fragment in which the double bond follows directly on the C1 atom. The olefin fragments are also characterized in that they have at least 14 carbon atoms, in particular 16 to 22 carbon atoms.
Particularly for starting materials which comprise the mixtures of different unsaturated oxygen-containing hydrocarbon compounds, but optionally also in the case of starting materials which contain just one oxygen-containing hydrocarbon compound which is a di- or triglyceride, several different olefin fragments are present. Hereinbelow, in this connection the olefin fragment with the largest molar fraction with k1 mol % is referred to as main component K1. Further olefin fragments are referred to as K2 to Kx and have a fraction of k2 to kx mol %. For determining the increased fraction of hydrocarbons of medium chain length, achieved according to the invention, only those olefin fragments are included here which are present at least in a fraction of 5 mol %. This proviso ensures that a simple determination of an increased fraction is possible, especially since olefin fragments with a low fraction exert only a slight effect on the product spectrum.
In step B) of the method according to the invention, the starting material is contacted with a porous carbon-based catalyst in the absence of oxygen at an elevated temperature in a conversion reactor. An elevated temperature is to be regarded here as being in particular a temperature of 200-800° C., preferably 300-600° C. and in particular 350-550° C. Under such conditions, the selected starting material gives a product mixture in which the fraction of hydrocarbons with medium chain lengths, i.e. in particular the fraction of at least one (optionally pure) hydrocarbon having 8 to 16 carbon atoms, has an increased fraction.
Finally, in a part step C), the hydrocarbon-containing product mixture is collected and fed to a separating device in which product separation takes place.
According to the invention, an increased fraction of hydrocarbons with medium chain lengths is understood as meaning that at least the fraction of one of the components with 8, 9, 10, 11, 12, 13, 14, 15 or 16 or more carbon atoms is increased in the product spectrum. In most cases, the fraction of at least two of these components is increased, often also that of three or more than three of these components. In particular, the component with y+3 carbon atoms occurs to an increased extent in the product spectrum (where y refers to the aforementioned formula of the olefin fragment). For the main component K1 and the other components K2-Kx, in each case the corresponding component is ascertained for this with y+3 carbon atoms, which accordingly serves in a way as a “marker”. According to the invention, an increased fraction can be detected on two different types, with the first alternative explained below being given preference on account of the simpler accessibility of the required data.
According to the first alternative, an increased fraction of hydrocarbons with medium chain lengths is determined as follows: firstly, for the main component K1 and each further component K2 to Kx optionally to be used (with more than 5 mol % fraction), the aliphatic product compounds β, which all have y+3 carbon atoms, are ascertained (for the main component thus the product compounds β1, for the further component K2 the product compounds β2 etc.). These are the marker compounds which have a particularly high fraction, where, as aliphatic product compounds β, besides the unbranched saturated compounds, only the monounsaturated product compounds of the product mixture are determined, and even only then if their weight fraction in the product mixture is more than 10%, based on the saturated unbranched product compound (lower contents complicate the determination method for the increased fraction unnecessarily and also do not significantly contribute to the result of this determination process, and are thus expediently omitted). The aliphatic product compounds βx with y+3 carbon atoms thus correspond to a hydrocarbon which arises formally by breaking the bond in the β position relative to the double bond on the side facing away from the chain end (in which case the product compounds are also included in which formally also a hydrogenation or isomerization of the double bond must take place). According to the invention, it has been established that compounds with such a chain length are formed to a particularly increased degree if the starting material specified above is used. The formal formation by breaking the bond in the β position, however, should not be understood herein as being restrictive, but is merely used for the definition of the product compounds βx.
Furthermore, for the main component K1 and each further component K2-Kx optionally to be used, also the corresponding product compounds δ, which all have y−1 carbon atoms, are ascertained (for the main component thus the product compounds δ1, for optionally present further components the product compounds δ2 etc.). Here as well, as above for the product compounds β, the unbranched saturated aliphatic product compounds are determined, as are the unbranched monounsaturated aliphatic product compounds if their molar fraction is more than 10%, based on the saturated aliphatic product compounds. The product compounds δ stand for a product component which is formed to a lesser extent or to a not increased extent and thus constitute an “antimarker”. They correspond to the segment which is formed if a bond separation formally takes place in the α position relative to the double bond on the side of the chain end of the product fragment. According to the invention, it was observed that these product compounds δ are formed in the starting material described above to a particularly low extent (based on the product compounds β).
According to the first alternative for determining the increased fraction, in a part step b), for all of the product compounds βx and δx thus determined, their molar fractions nβx and nδx present in the product mixture are then determined. For a starting material in which one main component and two further components are present, for the product compounds β1, β2 and β3, their fractions nβ1, nβ2 and nβ3 in the product mixture are thus determined and for δ1, δ2 and δ3, their fractions nδ1, nδ2 and nδ3 are determined.
According to the first alternative, in a part step c), finally all of the product compounds βx are used to calculate the averaged fraction nβ and for the product compounds δx the average fraction nδ, with the averaged fraction according to the formula being used for calculating the number average molar mass of a polymer, namely
      n    _    =            ∑              i        =        1            x        ⁢                  k        i            *                        n          i                .            If one main component and two further components are present, then for the three marker product compounds β the averaged fraction nβ is dependent on the molar fraction determined in each case of the main component and of the further components in the starting material and likewise for the antimarker, i.e. the product compounds δ, the averaged fraction nδ from the three antimarker product compounds.
An increased fraction of hydrocarbons with medium chain lengths (i.e. hydrocarbons having 8 to 16 carbon atoms) is present according to the invention if it applies that nβ>1.15*nδ, i.e. that the average fraction of the marker is at least 15% higher than the average fraction of the antimarker. Preferably, it applies that nβ>1.25*nδ and particularly preferably that nβ>1.5*nδ; the average fraction of the marker is thus at least 25% or 50% higher than that of the antimarker.
According to a second alternative, an increased fraction of hydrocarbons with medium chain length can also be detected as follows: as above in the first alternative, for a given starting material, the averaged fractions nβ are determined for the product compounds βx of the main component and all optionally present secondary components. Then, the starting material is hydrogenated by means of hydrogen/palladium catalyst in such a way that the fraction of the double bonds in the starting material is reduced by at least 95% (determination by reference to the iodine number). This material is reacted analogously to the method according to the invention under the same reaction conditions as the nonhydrogenated starting material to give a product mixture in which then, again for the same product compounds β as were determined for the nonhydrogenated starting material, the averaged fraction nβ# is determined. It then applies that nβ>1.15*nβ# (in the case of nonhydrogenated starting material at least 15% more of the marker is formed). Preferably, it applies that nβ>1.25*nβ# and particularly preferably that nβ>1.5*nβ# (i.e. at least 25% or 50% more of the marker is formed).
The method according to the invention offers for the first time the possibility of obtaining high yields of hydrocarbons with medium chain lengths, that is to say hydrocarbons of 8 to 16 carbon atoms, upon cleavage of compounds such as fats and oils over a porous carbon-containing catalyst by means of targeted selection of the starting material. The methods according to the prior art are essentially designed to cleave fats and similar substances to give long-chain hydrocarbons and/or lead to products in which merely an elimination of the carboxylate group of fatty acids is observed to an increased extent. The methods according to the prior art thus focus in particular on obtaining pure hydrocarbon compounds from a starting material by cleaving off the heteroatoms from the starting materials. A cleavage of carbon-carbon bonds often takes place only to a restricted extent or is even undesired if one disregards the fact that the reaction conditions may also be so drastic that only segments having 1 to 4 carbon atoms are formed to an increased extent. By contrast, it is not described how following elimination for example of the carboxylate group of a fatty acid to an increased extent (only) one further bond break takes place and accordingly an increased formation of hydrocarbons with medium chain lengths.
In the method according to the invention, it has also been recognized that as a result of the targeted use of double bond-containing starting materials, a product spectrum is obtained in which the hydrocarbons which arise as the result of the breakage of bonds that are adjacent to double bonds can be detected to an increased extent. In other words, it has been recognized according to the invention that by using double bond-containing starting materials, shorter products are obtained than when using the corresponding compounds which have a single bond instead of a double bond, i.e. are saturated.
Finally, it has been found according to the invention that particularly monounsaturated olefin fragments are especially suitable for achieving the desired product spectrum. With polyunsaturated compounds, it is often observed that under the conditions prevailing in part step B) a polymerization or other chemical reactions of the double bonds take place, meaning that during cracking of the polyunsaturated olefin fragments, no increased formation of hydrocarbons with medium chain lengths is usually observed.